Field of the Invention
This invention relates to fabricating multilayer plastic containers and more particularly to fabricating multilayer plastic containers capable of being sealed with metal closures.
In the manufacture of canned foodstuffs such as meat products as ham, corned beef hash, chili and beef stew, the containers, usually metal cans, are filled with the foodstuff, covered with a metal end closure and sealed.
One of the disadvantages of canning meat products in metal containers is that the presence of the food product may cause the interior of the can to corrode, which corrosion results in contamination and deterioration of the food product.
Attempts to substitute certain inert synthetic resin materials such as polyethylene and polypropylene for metal in the canning of foodstuffs have encountered the disadvantage that such resin materials are excessively permeable to gases such as oxygen and the permeation of oxygen into the container leads to undesirable discoloration and a depreciation in the taste and qualities of the foodstuff.
The high gas permeability characteristics of synthetic resins such as polyethylene has resulted in containers fabricated from such resins being rejected in the packaging of oxygen sensitive comestibles where, due to the chemical inertness of the resin, it might otherwise be employed to great advantage.
The art has devised a number of ways to increase the gas barrier properties of polyethylene and other thermoplastic resins. Included in these methods is the fabrication of the container from a laminate formed from a plurality of layers of thermoplastic material, one of the layers being formed from a thermoplastic resin which exhibits high gas barrier properties such as vinylidene chloride polymers and acrylonitrile polymers, e.g., U.S. Pat. Nos. 3,453,173, 3,464,546 and 3,615,308. Other methods include incorporating in the thermoplastic resin a filler material such as wood flour, inorganic mineral fillers such as clay or mica, e.g., U.S. Pat. Nos. 3,463,350 and 3,668,038 or a second resin such as nylon which has gas barrier properties, e.g., U.S. Pat. Nos. 3,093,255 and 3,373,224. Although these prior art methods are effective in improving the gas barrier properties of the thermoplastic resin, serious problems are encountered when it is attempted to seal the containers molded from these modified resins with metal closures.
In the conventional method of sealing metal containers with metal end closures, the closure is conventionally secured to the upper end of the container body by means of a double seam. In the double seaming operation, the end closure having a peripheral flange is applied to the open end of a container body which is provided with a flange integral with the container body and surrounding the open end thereof. The container flange is particularly constructed and dimensioned to receive and be interfolded with the end closure flange in a double seam to secure the closure to the container body. During the double seaming operation, the body and closure flanges are rolled together to form an interlocked double seam. During this operation, these flanges are squeezed or pressed together under considerable pressure to provide a hermetic seam. Although metal end closures may be readily double seamed to plastic containers formed from unmodified thermoplastic resins, when these same resins are modified by lamination or admixture with other plastic resins or by the incorporation therein of certain fillers, the flange portions of containers formed from such modified thermoplastic resins lack the flexibility and resiliency to withstand the severe stresses encountered during double seaming with the result that the upper end portions of the container body are caused to fracture or crack during the double seaming operation.
One method proposed by the art, i.e., U.S. Pat. No. 3,923,190, to manufacture containers having improved gas barrier properties which are sealable with metal closures is to mold the container from a composite billet having an outer peripheral area composed of a thermoplastic resin and an inner central core portion composed of the thermoplastic resin modified with a material which imparts reduced gas permeability to the resin. The composite billet is compression molded into a container wherein the billet core is formed into the side and bottom wall portion of the container and the peripheral area is formed into a flexible flange surrounding the open end of the container which can be double seamed to a metal end closure without fracture as the flange is devoid of any material modification which would otherwise reduce its flexibility.
One drawback to the process disclosed in U.S. 3,923,190 is that the construction of the outer peripheral area and core of the composite billet must be made with a high degree of precision which adds substantially to the manufacturing costs of the container molded therefrom.